I might suggest
Big Thunder Mountain because he can watch much of the ride beforehand & when riding it none of it is in the dark & there are no crazy loops, upside moves, or claustrophobic restraints. Similarly,
Splash Mountain is actually a calm ride until the big drop, which your son could watch beforehand.
Soarin' is a calm ride without restraints & speedy loops. You could also request that you sit in the lower portion of the ride so as to not go up so high on the first ride.
If those go well, your son may be up for the next step:
Space Mountain. More thrills and in the dark.
And if he decides he's ready for the big ones, perhaps your son might want to try the even bigger ones, but I'll caution you that these will be in the dark & have some crazy moves and bigger restraints:
Twilight Zone Tower of Terror,
Mission Space,
Expedition Everest, and
Rock 'n Roller Coaster Starring Aerosmith.
I included links to the rides, & many of them have videos your son can watch beforehand.